Your computer is fine. The fluctuation of framerate is common and it happens once in a while, not only for C2 games, but also other games using some other engines too.
Also, framerate is updated once every second only. Perhaps a frame got a tiny hiccup, that's why it got 57 fps. But this is negligible, I'd say.
I believe one of the possible reasons for this framerate drop is Garbage Collection: https://www.scirra.com/blog/76/how-to-w ... javascript